The proposed research attempts to characterize transport of solutes and water across the epithelium of proximal tubules. A combination of electrophysiological, microperfusion and ultrastructural techniques as well as flux methods will be used. 1. To study the electrical properties of transport systems at the level of the single cell membrane of the epithelium, as well as the electrogenesis of cellular and transepithelial potential differences in the isolated perfused proximal tubule. 2. To study the properties of ion transport systems at specific sites within the epithelium and across specific single plasma membranes, using measurements of changes in ionic activity inside the cell cytoplasm. 3. To study the pathways and compartments within a single proximal tubule epithelium, which contribute to the coupling between solute and solvent flow and which are important for fluid movement. 4. To study the ultrastructual detail of the amphibian proximal tubule and correlate structural membrane components and areas or intraepithelial compartments with transport functions. These studies are performed on amphibian isolated renal tubules because of the size of the cells and the stability of the preparation and as an adequate model which may lead to the understanding of the molecular membrane mechanisms which control salt and water balance in man in health and disease.